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GRACE operational issue: Planned switch-off of MWA

15 February 2017

The GRACE-A and B Microwave Assemblies (MWA) were switched-off on 5 February 2017, at 00:01 UTC, and 3 February 2017, at 22:00 UTC, respectively. The GRACE-B Accelerometer Instrument Control Unit (ICU) remains off.

Both activities are related to battery load reduction.

The switch-on of both MWAs is planned for approximately beginning of March 2017.

Background:

The GRACE orbit plane precesses at -1.117 degrees/day relative to the Sun, such that the Sun is in the orbit plane every 161 days. Due to the power system status and desire for longevity, this event will henceforth define a 161-day work cycle for science operations. As long as the beta_prime angle (angle between the orbit plane and the Earth-Sun line) is greater than 69 degrees, the satellite operates using power only from its solar array. For smaller beta_prime angles, the satellites operate partly using the arrays, and partly using the battery. When beta_prime is near zero (i.e. Sun is in the orbit plane), the battery may be used for as much a 40 minutes out of 90 minutes in each orbit.

Near beta_prime=0 events, the mission operations status depends on the battery health and operating environment.

Further information can be found on the University of Texas website.


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